Star Formation in Dwarf Galaxies of the Nearby Centaurus A Group

We present Halpha narrow-band imaging of 17 dwarf irregular galaxies (dIs) in the nearby Centaurus A Group. Although all large galaxies of the group have a current or recent enhanced star formation episode, the dIs have normal star formation rates and do not contain a larger fraction of dwarf starbursts than other nearby groups. Relative distances between dIs and larger galaxies of the group can be computed in 3D since most of them have now fairly accurately known distances. We find that the dI star formation rates do not depend on local environment, and in particular they do not show any correlation with the distance of the dI to the nearest large galaxy of the group. There is a clear morphology-density relation in the Centaurus A Group, similarly to the Sculptor and Local Groups, in the sense that dEs/dSphs tend to be at small distances from the more massive galaxies of the group, while dIs are on average at larger distances. We find four transition dwarfs in the Group, dwarfs that show characteristics of both dE/dSphs and dIs, and which contain cold gas but no current star formation. Interestingly the transition dwarfs have an average distance to the more massive galaxies which is intermediate between those of the dEs/dSphs and dIs, and which is quite large: 0.54 +- 0.31 Mpc. This large distance poses some difficulty for the most popular scenarios proposed for transforming a dI into a dE/dSph (ram-pressure with tidal stripping or galaxy harassment). If the observed transition dwarfs are indeed missing links between dIs and dE/dSphs, their relative isolation makes it less likely to have been produced by these mechanisms. We propose that an inhomogeneous IGM containing higher density clumps would be able to ram-pressure stripped the dIs at such large distances.Comment: 57 pages, 10 fi5gure

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism

The Colors of Dwarf Elliptical Galaxy Globular Cluster Systems, Nuclei and Stellar Halos

We present the results of a Hubble Space Telescope WFPC2 F555W and F814W survey of 69 dwarf elliptical galaxies (dEs) in the Virgo and Fornax Clusters and Leo Group. The $V-I$ colors of the dE globular clusters, nuclei, and underlying field star populations are used to trace the dE star-formation histories. We find that the dE globular cluster candidates are as blue as the metal-poor globular clusters of the Milky Way. The observed correlation of the dE globular cluster systems' $V-I$ color with the luminosity of the host dE is strong evidence that the globular clusters were formed within the the halos of dEs and do not have a pre-galactic origin. Assuming the majority of dE clusters are old, the mean globular cluster color- host galaxy luminosity correlation implies a cluster metallicity $-$ galaxy luminosity relation of $Z_{GC} \propto L_B^{0.22 \pm 0.05}$, which is significantly shallower than the field star metallicity - host galaxy luminosity relationship observed in Local Group dwarfs ($Z_{FS} \propto L^{0.4}$). The dE stellar envelopes are $0.1-0.2$ magnitudes redder in $V-I$ than their globular clusters and nuclei. This color offset implies separate star-formation episodes within the dEs for the clusters and field stars, while the very blue colors of two dE nuclei trace a third star-formation event in those dEs less than a Gyr ago.Comment: 39 pages, including 5 tables and 10 figures; accepted by the Astrophysical Journa

A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations

Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby galaxy clusters; even so, many of their basic properties have yet to be quantified. Here we present the results of our study of 4 Virgo dwarf ellipticals obtained with the SAURON integral field unit on the William Herschel Telescope (La Palma, Spain). While traditional long-slit observations are likely to miss more complicated kinematic features, with SAURON we are able to study both kinematics and stellar populations in two dimensions, obtaining a much more detailed view of the mass distribution and star formation histories. What is visible even in such a small sample is that dEs are not a uniform group, not only morphologically, but also as far as their kinematic and stellar population properties are concerned. We find the presence of substructures, varying degrees of flattening and of rotation, as well as differences in age and metallicity gradients. We confirm that two of our galaxies are significantly flattened, yet non-rotating objects, which makes them likely triaxial systems. The comparison between the dwarf and the giant groups shows that dEs could be a low-mass extension of Es in the sense that they do seem to follow the same trends with mass. However, dEs as progenitors of Es seem less likely as we have seen that dEs have much lower abundance ratios.Comment: 8 pages, 6 figures; to appear in the proceedings of the JENAM 2010 Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010); minor edits and references adde

Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452

We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar populations of the dEs in the vicinity of the red giant branch (RGB) tip. In both systems we find stars above the RGB tip, which we interpret as intermediate-age upper-AGB stars. The presence of such stars is indicative of extended star formation in these dEs similar to that seen in many, but not all, dEs in the Local Group. For AM 1339-445, the brightest of the upper-AGB stars have Mbol~-4.5 while those in AM 1343-452 have Mbol~-4.8 mag. These luminosities suggest ages of approximately 6.5+/-1 and 4+/-1 Gyr as estimates for the epoch of the last episode of significant star formation in these systems. In both cases the number of upper-AGB stars suggests that ~15% of the total stellar population is in the form of intermediate-age stars, considerably less than is the case for outlying dE satellites of the Milky Way such as Fornax and LeoI.Comment: 18 pages, 19 figures, A&A accepted; high resolution version available from: http://www.eso.org/~mrejkuba/CenA_dEs_I.pd

Ethnic identity in West Africa. Historical perspectives on America\u27s discourse about ethnicity

Political Correctness and Affirmative Action are core issues of America’s difficult search for cultural identity. However, a policy of differential treatment of categorized ethnicities is likely to cause complex and dynamic identities to become static and one-dimensional. This in turn is a factor of social destabilization. The basic assumption of ethnicity as a fixed social or racial category is not only an over-simplification, but a policy focussing on group rights changes the character of the groups themselves and even stimulates growing isolation between them. The paper’s argumentation is based on observations on the historical process of group identity-forming in Western African societies. They provide the example of how ethnic identity is a matter of dynamic constellations, of change, of interaction, of conscious and unconscious negotiation and choice, both on the group level and on the individual level.Hintergrund der Argumentation des Autors "ist die Bewegung für \u27Political Correctness\u27 und \u27Affirmative Action\u27 in den USA. Als eine ihrer Konsequenzen impliziert sie den grundsätzlichen Antagonismus zwischen verschiedenen kulturellen Identitäten, der im gegebenen politischen Kontext virulent wird und den anderen nicht mehr an der eigenen Kultur teilhaben läßt bzw. zur Wahl für oder gegen zwingt. Gestützt auf eigene empirische Untersuchungen und ethnographische Feldforschung über die Normalität mehrsprachiger und plurikultueller Enkulturation in traditionellen afrikanischen Gesellschaften vertritt er das Konzept nicht-determinierter, dynamischer, nicht-ausschließlicher kultureller Identitäten." (DIPF/Orig.

Fruitgrowers towards a new approach to enhance biodiversity in organic orchards

In the frame of the project “Arbeitsnetz zur Weiterentwicklung der Anbauverfahren des ökologischen Obstbaus” (BOEL-project Nr. 03OE178) a group of fruitgrowers and advicers started to discuss new measures and new concepts to enhance biodiversity in organic orchards. Strips with flowering plants that can be integrated in the normal mulching system and thus, do not originate vole problems, are actually tested on the farms. Other measures as single bushes at the top and end of the rows are in discussion. If such measures are to be integrated successfully in the practice of organic fruit growing, the impulse must spring from growers themselves and their requirements regarding the different risks must be considered. Otherwise, an acceptance in practice is not very probable. This activites are reported here since we want more people to join us